Conventionally the power from wind turbine systems is transferred mechanically, either directly or by a rotational speed changing gearbox to a generator. The generator can be connected to the electric grid or net so that it is forced to rotate at a synchronous frequency and thus a fixed speed, incurring that the generator will act as a motor if the turbine is not supplying an appropriate level of mechanical torque and power, and thus the net may drive the generator and turbine through the gearbox. It is known in the background art that the angle of the turbine blades can be varied by a hydraulic system to either obtain maximum power from the turbine at a given wind speed or limit the power to a desired level if the wind speed is above a design limit. As the turbine speed is kept constant by the generator the blade angle can be varied in order to maintain maximum efficiency and hence maximum power output from the turbine over a range of wind speeds. A disadvantage with controlling the generator speed by adjusting the pitch is the fact that the response time of the system's adaptation to changing wind speeds may be slow.
Wind and water turbines are currently important devices for generating energy. As a consequence of the deficiencies of prior art hydraulic transmissions for wind power plants, one state of the art in designing such plants, is the use of a turbine generator assembly with a high ratio gearbox (gearbox) at the top of the turbine tower. This design introduces a series of structural challenges, which has limited the size and output of the plants and impaired the efficiency. The use of gearboxes has increased the weight and the costs and the noise associated with wind power plants. Further, it has enforced the use of single unit plant, thus increasing the costs and the weight of each unit.
An alternative method for transmitting the turbine power uses a hydrostatic transmission system consisting of a hydraulic pump and a hydraulic motor. The hydraulic pump is driven by the turbine which supplies flow to a hydraulic motor that is connected to the generator. The appropriate selection of the pump and motor displacements provides the desired speed ratio between the turbine and the generator and using a variable motor displacement allows the speed ratio to be varied.
In U.S. Pat. No. 4,503,673 (Schachles, 1979) the hydraulic pressure generated by the turbine pump is sensed and compared with a datum value that is varied with the velocity of the wind. If the pressure is lower than the set value, the motor displacement is increased, thus increasing the turbine speed until the actual pressure and the set pressure are equal. Thus as the wind speed is increased, so the turbine speed increases in the way that the datum value is varied with the wind velocity in order to create a constant tip speed ratio (TSR).
There are some advantages of measuring the turbine rotational speed and using this as an input to a control system according to the invention when compared to the system using pressure measurements for controlling the generator speed as described in U.S. Pat. No. 4,503,673. The advantages include:                Improved accuracy of the operating point for maximum efficiency. This is because of the low rate of variation in the hydraulic pressure with changes in turbine speed, for a given wind speed, which could cause uncertainty in its operation. It is also likely that the graphical relationship is concave upwards which could worsen this problem. Using turbine speed control the speed that creates maximum turbine efficiency can be more precisely defined.        As a result of a) and also because of the way in which the hydraulic pressure arises in the system, it is likely that there would be problems in providing an acceptable dynamic response for a pressure control system. In this event and to avoid instability, the value of system controller gain would have to be set at a level that would further compromise its steady-state accuracy.        
Various power plants with one or more power units have been proposed with a hydraulic transmission between the power unit(s) and an electric generator. The one or more power units may be wind turbines, water turbines, e.g. for tidal power plants or wave power plants. Such transmissions will allow multiple driving units to operate one hydraulic motor connected to an electric generator, a pump or another working machine.
Wind power plants with a wind turbine powering a hydraulic pump with a transmission to a hydraulic motor have long been known.
In German patent publication 30 25 563 (Calzoni 1980) a hydraulic transmission for wind power plants is proposed, where the starting is controlled manually by a shunt valve and the maximum operating pressure is limited by a pressure control valve. No control is provided for maintaining a fixed hydraulic motor speed or optimising the operation of the turbine.
Japanese patent application 61212674 (Matsushita Seiko 1986) describes a wind power plant with a hydraulic pump and electric generator assembly arranged at the foundation of the wind turbine tower.
WIPO-publication WO 94/19605 (Gelhard 1994) describes a wind power plant with multiple turbine units which can be connected to operate one hydraulic motor generator assembly arranged at the foundation of one of the units.
From U.S. Pat. No. 4,503,673 (Schachle et al 1985) a wind power plant is known to be connected to a hydraulic pump and a variable displacement motor connected to drive an electric generator. When operating at a constant speed the flow to the motor is proportional to its displacement. For the speed of the turbine to increase, the displacement of the motor must increase to permit an increase in the flow rate through the motor.
WIPO-publication WO 03/098037 (MLH Global 2003) describes a wind turbine with a variable displacement, pressure compensated hydrostatic transmission. A major objective of this device is to control “overrunning” loads. For this purpose, means for varying the displacement of the transmission in response to the variations in pressure within the oil circuit is proposed. The proposed displacement control is operating during the start up and not for being used during normal operation.
In connection with wind turbines operating an electric generator at a fixed speed ratio, various wind turbine pitch control systems are known. In WIPO-publication WO 99/07996 such a control system is described, which is not suitable for a hydraulic transmission.
From U.S. Pat. No. 4,622,473 (Curry, 1986) it is known to let a multitude of wave operated hydraulic pumps power a hydraulic motor, which in turn powers an electric generator. This system has no hydraulic control system for maintaining any particular frequency.
French patent application FR-2755473 describes a hydraulic transmission system for use in wind turbines. The transmission is controlled by a closed loop servo system which has a speed detector measuring the rotational speed of the generator, and a regulator receiving the output from this detector and using it to control the angle of inclination of the plate in the variable flow pump. The control system does not measure turbine rotational speed or wind speed, and cannot be used for improving efficiency based on an optimised tip speed ratio.
US patent application US2005/0194787A1 describes a control system with three control levels for a wind power plant with a hydrodynamic gear where the wind power plant is connected to an electric grid. A controller controls an angular position of the rotor blades and/or controls a setting of the reaction member of the hydrodynamic speed transformer and/or controls the power electronics of the generator. The controller is provided with predetermined setpoint characteristics depending on operating states of the wind power plant and/or the electric grid or characteristics of the wind. The power transmission as such is a mechanical gear.